1. Field of the Invention
The present invention relates to a far electromagnetic field estimation method and apparatus for measuring an electromagnetic field generated by a radiation source of electromagnetic waves, such as an electronic apparatus, near the radiation source and estimating, on the basis of the measured electromagnetic field, an electromagnetic field at a virtual observation point located farther from the radiation source, and to a near electromagnetic field measurement apparatus suited for implementing the far electromagnetic field estimation method.
2. Description of the Related Art
A test for measuring radiated emissions radiated from an electronic apparatus or the like is typically performed according to an internationally-defined test condition and test method, mostly by measuring the radiated emissions at a distance of 10 m or 3 m from the radiation source of the radiated emissions. A test for measuring radiated emissions in the frequency band of 30 MHz to 1 GHz is typically performed with an object to be measured, or the radiation source, arranged on or above a ground plane (metal floor surface).
In general, a test for measuring radiated emissions is performed in an open site or in a radio anechoic chamber that satisfies internationally-defined site compatibility. A radio anechoic chamber is constructed by attaching a radio wave absorber to the wall surfaces of a shield room. Depending on factors such as the intended use of the radio anechoic chamber and the frequency of electromagnetic waves in use, the size and shape of the shield room and the type of the radio wave absorber are selected. Typical radio anechoic chambers include a 10-m test range radio anechoic chamber and a 3-m test range radio anechoic chamber. A 10-m test range radio anechoic chamber has a length of approximately 20 m to 30 m, a width of approximately 10 m to 20 m, and a height of approximately 7 m to 12 m. A 3-m test range radio anechoic chamber has a length of approximately 7 m to 11 m, a width of approximately 4 m to 7 m, and a height of approximately 5 m to 7 m. Such radio anechoic chambers are used as appropriate according to the distance required for the test for measuring radiated emissions.
Disadvantageously, radio anechoic chambers need a relatively large length, width and height as described above, need a large-scale building and facility, and increase the total investment and the maintenance and operation costs for the building, facility, and various necessary equipment.
A small-scale apparatus that can measure radiated emissions in a smaller space is thus demanded. To meet such a demand, there have been proposed an apparatus for measuring a very near electromagnetic field and an apparatus for measuring a quasi-near electromagnetic field intended for a printed circuit board or the like, and a method for estimating a far electromagnetic field from a measured near electromagnetic field by using Love's equivalence theorem or the like.
For example, JP 2004-069372A describes a method including setting a virtual rectangular solid or circular cylinder to surround an object to be measured, measuring an electromagnetic field in such a manner as to scan the surfaces of the rectangular solid or circular cylinder and, on the basis of the measured electromagnetic field, estimating an electromagnetic field in a position distant from the object. JP 2004-069372A further describes that only five of the six faces of the virtual rectangular solid may be scanned with the remaining one as a ground plane.
Masataka MIDORI et. al, “A Fundamental Study on Estimation of 3 m Test-range from Near-field Electromagnetic Field Measurement,” The Institute of Electronics, Information and Communication Engineers (IEICE) technical report, EMCJ2013-11 (2013-05) describes a method including selecting a dipole antenna as the object to be measured (radiation source), and determining a far electromagnetic field at an observation point in the following manner under the condition that the dipole antenna is arranged on a ground plane. According to the method, the amplitudes and phases of tangential components of a near electromagnetic field on six planes set to surround the object to be measured are initially measured. Next, assuming that the ground plane is an infinite conductor flat plate, a mirror image of the observation point formed by the conductor flat plate is assumed as a mirror image observation point. Electromagnetic fields at the two points, the observation point and the mirror image observation point, are then estimated. The two estimated electromagnetic fields are then vectorially added to determine the far electromagnetic field at the observation point.
According to the method described in JP 2004-069372A, the virtual rectangular solid or circular cylinder is set to surround the object to be measured without considering a ground plane. Then, the surfaces of the rectangular solid or circular cylinder are scanned, or five of the six faces of the virtual rectangular solid are scanned with the remaining one assumed as a ground plane. Thus, the method does not take account of the effect of a ground plane. If such a method is applied to a test performed under the foregoing condition that the object to be measured is arranged on a ground plane, it can be easily imagined that the estimated electromagnetic field differs greatly from the actually measured electromagnetic field due to the effect of the ground plane.
According to the method described in the literature of MIDORI et. al, it is necessary to measure the near electromagnetic field at the surface under the object to be measured. In actuality, however, it is difficult to float the object to be measured in the air, and therefore the near electromagnetic field at the surface under the object to be measured is difficult to measure. Such a method is thus difficult to apply to a test for measuring radiated emissions. In addition, the method needs calculation processing for estimating the electromagnetic fields at the two points, the observation point and the mirror image observation point, and calculation processing for vectorially adding the electromagnetic fields estimated at the two points. This causes the problem that a lot of calculation processing time is needed.